Manufacturing industries are constantly searching for ways to produce high-performing products with lower-cost materials, while also improving product and manufacturing efficiencies. These goals can sometimes be met through original equipment manufacturers that drive the demand for such improvements through supplementary materials providers. In industries that rely upon electrical interconnections, charge collection tapes are a class of supplementary materials that can provide an economical way to make critical electrical interconnections.
Charge collection tapes have been used as grounding tapes, for EMI shielding, and interconnects in many different devices, such as mobile phones and other portable electronics. In recent years, charge collection tapes have found utility in solar applications and other applications that make use of photovoltaic cells.
Charge collection tapes consist of a metal foil carrier and an adhesive. Many charge collection tapes are high-performance adhesive tapes capable of withstanding temperature extremes, thermal cycling, damp heat exposure, and a number of other environmental stresses.
In solar applications, charge collection tapes are being investigated as cell stringing interconnects between photovoltaic cells to replace solder and silver filled electrically conductive adhesives (Ag-ECA), as well as for terminal bussing systems to collect charge from stringing ribbon or foil where a simple lamination can potentially replace the need for soldering to the bus bar foil. Charge collection tapes have also been useful in thin film designs in which the tape collects charge directly off the photovoltaic cell and transfers it to a second foil that carries the charge to a junction box outside the module. In all of these examples, the charge collection tape becomes an integral part of the photovoltaic design, and reliable adhesive and high-performing electrical interconnects are important for avoiding issues related to power loss or complete failure of a photovoltaic module.
The use of an adhesive laminated to copper foil as a charge collection tape is well known, including for use in solar applications. Copper foil is highly conductive, but has become increasingly expensive. Copper is also susceptible to oxidation and corrosion. In solar applications, that can be problematic because in many cases, the charge collection tape foil surface is in direct contact with the module encapsulant such as ethyl vinyl acetate (EVA) or polyvinyl butyral (PVB). Photo and thermal degradation of the encapsulant can generate corrosive species, such as acetic acid in the case of EVA, which can corrode the copper surface. This corrosion, while having a generally limited effect on performance, creates an undesirable aesthetic that can be commercially unacceptable by having an adverse effect on consumer interest in adopting solar technology.
Tin-coated copper foil generally performs better in these cases and is also known and widely used to form charge collection tapes. The tin protects the copper from oxidation and corrosion. In addition, the aesthetics of oxidized tin is quite often more appealing than that of copper. This is particularly true of copper foil treated with temporary antioxidants such as benzotriazole and other azole derivatives. These compounds form a myriad of colors when exposed to high temperature creating a visually unappealing foil surface, which can again have an adverse effect on consumer interest.
Tin coated copper foil has also been used successfully in conventional silicon wafer solar cells. Its proven track record has made it the foil of choice for most charge collection tapes for long life photovoltaic applications.
Unfortunately, however, the cost of copper and tin-coated copper foil carriers are quite expensive. In many cases the foil carriers make up a majority of the cost that goes into the charge collection tape. There is a need, therefore, to provide charge collection tapes that overcome these and other drawbacks, but which still provide high conductivity and good aesthetics.